Semi Floating Rotors = You have a center piece and a separate outer part that the brake pads touch. The two are connected by a bunch of round "buttons" that allow a small amount of movement between the two.

Full Floating Rotors = The braking surface is relatively loose so that it can move around to hopefully become in perfect alignment with the pads. Full floating rotors are less likely to pulse than solid or Semi Floating rotors. Full floating rotors make noise at low speeds as the parts move around, since they're kind of loose.